The present invention generally relates to channel estimation, and particularly relates to channel estimation in a transmit diversity environment.
Transmit diversity techniques, broadly referred to as multi-antenna transmission schemes, are of increasing interest in wireless communication systems, primarily for their promise of higher data transmission rates and/or the ability to serve a greater number of users. Such schemes include various Multiple-Input-Single-Output (MISO) and Multiple-Input-Multiple-Output (MIMO) approaches, and may involve various forms of Space-Time-Coding (STC) techniques.
Not surprisingly, however, the use of transmit diversity brings with it attendant increases in system complexity. For example, the use of transmit diversity schemes typically exacerbates the problem of channel estimation in wireless communication systems, because channel estimates typically must be made for multiple transmit antennas rather than just one. In general, full realization of transmit diversity benefits requires the estimation of channel paths between each transmit/receive antenna pair.
One difficulty with accurately generating reliable channel estimates for particular transmit antennas or, more generally, for subsets of one or more transmit antennas, involves the amount of antenna-specific pilot information available to the receiver. Suitably reliable channel estimates for each transmit antenna could be obtained in a relatively straightforward manner if each transmitting antenna or, more generally, each subset of one or more associated transmit antennas, provided a rich amount of pilot information. However, the transmission of pilot information comes at the expense of data transmission—i.e., transmission resources used for pilot information generally represent resources not available for data transmission. Thus, there is a tension between the desire to support good channel estimation at targeted receivers, which is after all required for acceptable receiver performance, and the desire to maximize the amount of transmission resources allocated for the transmission of data (user traffic and control signaling).
The implementation of transmit diversity tends to complicate matters because the targeted receivers generally need enough antenna-specific pilot information to make good channel estimates for each subset of transmit antennas. That need is at odds with current and developing approaches, which tend to divide or otherwise split the total amount of pilot information being transmitted across the different transmit antennas being used for transmit diversity.
For example, Orthogonal Frequency Division Multiplexing (OFDM) signals, such as WiMax signals defined by the IEEE 802.16 standards, define a total number of narrow band subcarriers, each subcarrier having a different frequency. Different subsets of subcarriers are used at different times to transmit information to users, with some relatively small number of subcarriers designated as pilot subcarriers, with the remaining subcarriers designated as data subcarriers. With one transmit antenna, the number of pilot subcarriers generally is sufficient for good channel estimation at the targeted receivers. However, when the total number of pilot subcarrier designations is split or otherwise shared across multiple transmit antennas, the amount of antenna-specific pilot information is correspondingly reduced.
That condition generally is not a problem at least for the downlink “preamble” portion of WiMax signal frames, but it is problematic for the data or payload portion of those frames. More specifically, the preamble portion of a WiMax frame often is used as a “broadcast” type transmission, allowing reception at targeted receivers. As such, it may be undesirable to adopt a multi-antenna transmission scheme for the preamble that may be incompatible with certain receivers. One approach is the use of transmit delay diversity for preamble transmission, wherein the same signal is transmitted from different transmit antennas at different times, with the overall “channel” being a composite of the involved transmission paths.
In contrast, the different bursts within the data portions of WiMax frames generally are targeted to individual receivers and thus can be tailored to the diversity processing capability of those receivers. Therefore, the payload or data portion of the downlink transmission may use another form of multi-antenna transmission, i.e., something other than transmit delay diversity, to obtain higher data throughput and/or to serve more users. However, doing so decreases the number of antenna-specific pilot symbols available to the targeted receivers, and thereby complicates the task of making reliable channel estimates for the data portion of the WiMax signal.